1. Field of the Invention
The invention relates to the field of gamut boundaries for color devices, and more particularly relates to performing gamut operations using a descriptor which represents a gamut boundary of a color device.
2. Description of the Related Art
A gamut boundary descriptor (GBD) is a description of the gamut boundary of a color device. In general, a GBD is represented as a closed surface in a 3-dimensional color space. For example, a GBD can be represented as a set of interconnected planar triangles in the 3-dimensional color space.
GBDs are employed by different gamut mapping algorithms (GMAs), which are algorithms that map colors between color devices while taking into account any differences between the color gamuts of the color devices. Each GMA is typically optimized for one particular application or directive, such as preserving colorimetric value, emphasizing saturation, or making the reproduction perceptually pleasing. It should be noted that GMAs typically operate under the assumption that the gamut boundary has been calculated and represented in some way, so that geometrical calculations can be performed.
There are several common operations required by GMAs that employ GBDs. One such operation is the check gamut operation, which determines whether a given color in color appearance space is inside the gamut boundary of a color device. Another operation is the get cusp operation, which determines a point on the gamut boundary of a color device with the largest chroma. Yet another operation is the ray intersect operation, which determines the intersection(s) with the gamut boundary of a color device with a given ray representing a directional half line.
Different approaches have been taken for implementing the foregoing operations. For example, one approach is taking hue slices of a convex hull representation of the gamut. In a color appearance space that supports the attribute of a hue, a hue slice can be defined as the intersection between a hue plane, which is a plane in the color appearance space wherein all points on it have the same hue, and the gamut boundary. It should be noted that a full hue plane actually contains 2 hues that are 180 degrees apart, so the above intersection between a gamut boundary and a hue plane actually occurs within the half-plane having the specified hue.
The convex hull representation of the gamut may provide a geometric depiction of a GBD that matches an intuitive visualization of the GBD. Moreover, a convex hull slice of the gamut may provide a unique intersection point with a ray emanating from a point on the neutral axis of the color device. However, the convex hull representation is not without its drawbacks.
First, color devices such as display devices may have gamuts that are not convex. Since the convex hull representation of a gamut assumes a convex shape, the actual gamut may deviate from the convex hull representation, leading to inaccurate results.
In addition, since a hue slice is taken along a constant hue, and since the hue slice is taken along a half plane that corresponds to the hue angle, the hue slice may contain features which the convex hull tends to overestimate. For example, color devices such as CRT devices may produce hue slices with “islands”, which the convex hull would include, along with all the void space between them. Similar difficulty arises for color devices such as color printers that produce hue slices where part of the neutral axis of the color device is missing.